A closed loop current controller for an electric actuator typically takes the form shown in FIG. 1. Using vector control in response to a requested target current, a voltage is applied to each phase winding of the motor and the resultant generated currents measured. The measured currents are then used by a controller, typically a PI controller, to control to the requested voltage to achieve the target current. A signal which gives the electrical position of the motor is also required so that the voltages can be applied at the correct phases at the correct times. In the system of FIG. 1 a position sensor is shown but the present invention is equally applicable to a position sensorless system, where the position signal is estimated from other sensor measurements.
Typically for a 3 phase motor the driver comprises a 6 FET bridge arranged to apply a known voltage to the motor terminals in a PWM pattern to achieve the desired closed loop current. Due to the PWM switching patterns that are applied to the motor to produce the desired voltage, drive stage non-linearities in the form of additional unwanted harmonic frequencies can be introduced as shown in FIG. 2.
Our earlier application PCT/GB2011/050748 discloses a motor control system in which the level of distortion at the distortion frequency is measured and a correction signal input to the controller which can then correct the applied voltages to compensate for the distortion. To improve measurement of the distortion, the current component at the distortion frequency is isolated from the current component at the fundamental frequency, by applying a frequency shifting transformation to transform the distortion frequency to a DC component, and then filtering out the fundamental frequency component using a low pass filter. At high motor speeds the difference between fundamental and distortion frequencies is sufficient for this to be achieved effectively. However at low motor speeds the difference between the two frequencies is lower, and effective filtering becomes much more difficult.